Method of making a variable resistor

ABSTRACT

A variable resistor having a lead screw is provided with front and rear journals for the lead screw which are simultaneously formed by ultrasonically vibrating the lead screw and forcing a conical tip thereof to penetrate the rear wall of the resistor housing while the head of the lead screw is forming a front journal. A fixture is inserted in the housing to hold the lead screw in accurately aligned position while the journals are formed. Thereafter the housing is ultrasonically deformed to hold the resistance carrying substrate assembled with the housing and a potting agent is used to seal the substrate to the housing.

nited States Patent 1 ill, Jr.

[ METHOD OF MAKING A VARIABLE RESISTOR [75] Inventor: Robert D. Hill, Jr., West Covina,

Calif.

[73] Assignee: Spectrol Electronics Corporation,

City of Industry, Calif.

[22] Filed: Jan. 28, 1972 [21] App]. No.: 221,628

[52] US. Cl. 29/613, 29/610, 29/DIG. 46,

29/437, 156/73, 264/23, 338/180, 338/184 [51] Int. Cl 1101c l/02, H010 17/00 [58] Field of Search 29/610, DIG. 46,

Primary Examiner-Charles W. Lanham Assistant Examiner--Victor A. DiPalma AttorneyI-larry G. Martin, Jr. and .1. Raymond Curtin et a1.

[57] ABSTRACT A variable resistor having a lead screw is provided with front and rear journals for the lead screw which are simultaneously formed by ultrasonically vibrating the lead screw and forcing a conical tip thereof to penetrate the rear wall of the resistor housing while the head of the lead screw is forming a front journal. A fixture is inserted in the housing to hold the lead screw in accurately aligned position while the journals are formed. Thereafter the housing is ultrasonically deformed to hold the resistance carrying substrate assembled with the housing and a potting agent is used to seal the substrate to the housing.

5 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION This invention relates to variable resistors especially of the type frequently called trimmer potentiometers.

Manufacturing cost is a significant consideration in the design of trimmer potentiometers because they are usually relatively inexpensive. However, with the development of improved resistor materials and wiper elements, trimmer otentiometers can be inexpensively made which approach precision performance specifications. It, therefore, becomes increasingly important to simplify the mechanical design of the trimmer to reduce manufacturing costs without, however, reducing the exacting tolerances required for a precision product.

One area which has proved troublesome in prior trimmer potentiometer designs has been that of forming a smoothly operating, inexpensive rear journal for the lead screw which is accurately aligned with the front journal. Failure to achieve good alignment may result in poor setability and increased electrical noise in the unit. Rear journals have evolved from complex and expensive ball bearing designs through a wide variety of less expensive but often less satisfactory interfitting elements. In an attempt to simplify the construction of trimmer potentiometers, it is known to form a front journal and seal by ultrasonically vibrating a metal lead screw in the resistor housing. However, it has been found that this technique may result in a unit having excessive electrical noise, backlash and setability problems. The cause of much of this trouble has been found to be due to wobble of the lead screw caused by a poorly supported or inaccurately aligned lead screw because of the rear journal not smoothly and tightly engaging and supporting the shaft.

It is the principal object of this invention to provide a simple and inexpensive method of making a properly aligned, smoothly operating rear journal for the lead screw without employing any parts which have to be assembled in the resistor housing.

SUMMARY OF THE INVENTION A housingmember for a variable resistor is formed with front and rear thermoplastic end walls spacedapart by a side wall. A cylindrical aperture is formed in the front end wall of the housing. An adjustment member, such as a lead screw, having an annular rib on its head, is inserted from the exterior of the housing memher through the cylindrical aperture in the front end wall. A fixture is inserted into the housing to accurately position the lead screw therein. The lead screw is then ultrasonically vibrated and forced further into the housing member until its rear end penetrates the rear end wall of the housing to form a rear journal. Simultaneously, the rib on the head of the lead screw penetrates further into the cylindrical aperture in the front wall of the housing to form a front journal which is accurately aligned in the desired location with respect to the rear journal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a longitudinal cross-section through a completed variable resistor made in accordance with this invention;

FIG. 2 illustrates a cross-section through the variable resistor illustrating the method of inserting the lead screw member into the housing member;

FIG. 3 is a cross-sectional view taken on line 3-3 of FIG. 2; and

FIG. 4 is a cross-sectional view illustrating the further assembly step of securing the substrate member with the housing member.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a completed variable resistor 10 of the type commonly referred to as a trimmer potentiometer. A five'sided hollow housing member 11 is assembled with a lead screw type adjustment member 12, a substrate member 13 and a wiper member 14 as illustrated in the drawing. A substrate member 13, carrying a resistor element and a collector element, forms a closure for the open side of the housing member and a pot ting agent 15, such as epoxy resin, is solidified over the exterior surface of the substrate member to seal it with the housing member. The front and rear ends of lead screw adjustment member 12 are journaled for rotation in the housing member so that rotation of the lead screw causes wiper member 14 to traverse the resistance and collector elements which are disposed on the interior surface of substrate member 13 in a manner well known in the art. Terminals l6, l7 and 18 project from the exterior of the variable resistor for making electrical connection with the ends of the resistance element and the wiper member.

One end wall 20 of the housing member forms a front wall which is spaced from the other rear end wall 21 by top side wall 22. Front end wall 20 has a cylindrical aperture 23 extending therethrough. Housing 11 is made of a thermoplastic material and housing member 11 is preferably molded with a longitudinally extending concave groove 36 facing the interior of the housing which assists in accurately aligning the lead screw in the housing during assembly thereof.

Lead screw 12 is formed of metal and has a head 30, a threaded shank 31 and a conically pointed tip 32 on the rear end thereof. A circular cross-section portion, such as one or more annular ribs 33, is formed on the head end of the lead screw and preferably has an outer diameter greater than the inner diameter of cylindrical aperture 23 in the front wall of housing member 11.

Lead screw 12 in inserted tip first through cylindrical aperture 23 in the front end of the housing member until ribs 33 engage front end wall 20 or until conical tip 32 engages the interior surface 24 of rear end wall 21 which prevents further manual insertion. The partially assembled housing and lead screw members are then inserted in housing fixture 40 which securely embraces the housing member. Housing fixture 40 has end walls 41 and 42 which form a cavity 43 to hold the housing member from motion in either directionL'The housing fixture is also provided with a pair of alignment pins .44 and 45 to hold a second fixture 50 in accurately ing member and accurately aligned with the housing fixture member by means of pins 44, 45. Fixture 50 is formed with a concave groove 53 which rests on top of the threaded shank 31 of lead screw member 12 and forces the lead screw to rest in the bottom of concave groove 36 of the housing member. After fixture 50 is assembled with housing fixture 40, lead screw member 12 is captured between groove 53 in fixture 50 and groove 36 and housing 11 so that the lead screw is very accurately aligned with respect to the housing member.

An ultrasonically vibratable tool 60 is then brought into engagement with head 30 of the lead screw member and axial force is applied between tool 60 and housing fixture 40 to force the lead screw member further into the housing member. The ultrasonic vibration of the lead screw causes the thermoplastic material of end wall 21 on housing member 11 to soften and flow about the region of conically pointed tip 32 on the lead screw. Similarly, the ultrasonic vibration of ribs 33 causes the thermoplastic material of end wall 20 on the housing member to soften and flow about annular ribs 33 on the head of the lead screw. When tool 60 is removed, the thermoplastic material of the end walls of the housing hardens and a front and rear journal is formed for the lead screw. Since the lead screw is accurately positioned with respect to the housing by the housing fixture and the lead screw fixture, precise alignment of the lead screw in the housing is achieved without the necessity of employing hand assembled parts to form the journals.

Thereafter, fixture S is withdrawn from housing member 11 and wiper member 14 is positioned within the housing. Substrate member 13 is then assembled over the open end of the housing member so that it rests on shoulders 61 and 62 thereof. Substrate member 13 substantially closes the open side of housing member 11. A second ultrasonic tool 65 is then brought into engagement with portions of the side walls of housing member 11 to deform a portion of the side wall into engagement with the exterior surface 66 of the substrate member as shown in FIG. 4. This operation is performed at a plurality of locations such as 68 to temporarily secure the substrate member to the housing member.

Housing member 11, having substrate member 13 secured thereto, is then removed from housing fixture 40 and a layer of a potting agent 15-such as epoxy resin is flowed over the exterior surface 66 of the substrate member and cured so that it solidifies to form a tight seal between the substrate member and the housing member.

While it is preferred to use a lead screw or other adjustment member having a conical tip thereon for penetrating the rear wall of the housing, other shapes which are circular in cross-section may also be employed. While this invention has been described with respect to a trimmer potentiometer having a linearly movable wiper member, the principles of this invention may also be applied to other types of variable resistors. For example, another common type of trimmer potentiometer utilizes a worm gear driven by a threaded adjustment member which can be journaled in its associated housing in a manner similar to that described herein.

This invention enables a variable resistor to be manufactured inexpensively because of the simple journal construction. Furthermore, this invention enables an accurately aligned adjustment member to be formed with a rear journal which is free from wobbling or stresses and which has a very smooth rotational action. These factors contribute to a relatively high degree of setability with freedom from backlash and electrical noise due to wobbling of the adjustment shaft which are often associated with other types of journals. This invention, therefore, enables an inexpensive construction to be manufactured which approaches precision standards of performance.

It will be understood that this invention may be otherwise embodied within the scope of the following claims.

I claim:

1. In a method of making a variable resistor having a housing member formed with a pair of spaced end walls connected to a side wall, one of said end walls having a cylindrical aperture formed therein, and a metal, adjustment member formed with a circular crosssection head, a threaded shank and a circular crosssection tip on the end opposite the head thereof, the steps including:

A. inserting the adjustment member, tip first, from the exterior of the housing member through the cylindrical aperture in the one end wall of the housing;

B. inserting a fixture into an open side of the housing member opposite said side wall into engagement with the adjustment member to position the adjustment member in the desired position between the fixture and the side wall of the housing member;

C. ultrasonically vibrating the adjustment member relative to the housing member while simultaneously exerting axial force between the adjust ment and housing members to drive the head of the adjustment member into the cylindrical aperture in said one end wall of the housing member until the circular cross-section tip of the adjustment member penetrates a substantial extent into the other of said spaced end walls of the housing member, thereby causing the thermoplastic material of said one end wall of the housing member to soften and flow about the head of the adjustment member and causing the thermoplastic material of said other end wall of the housing to simultaneously soften and flow about the tip of the adjustment member;

D. discontinuing the ultrasonic vibration and allow ing the thermoplastic housing material to solidify about the region of the head and about the tip of the adjustment member whereby a front and a rear journal are simultaneously formed to rotatably support the adjustment member in the desired position in the housing member; and

E. thereafter withdrawing the fixture from the housing member.

2. A method of forming a variable resistor as defined in claim 1' including the step of forming a concave groove in said side wall of the housing member for as sisting in locating the adjustment member in the desired position relative thereto.

3. In a method of making a variable resistor having a hollow housing member with an open side formed by a pair of spaced end walls connected by three side walls, one of said end walls having a cylindrical apcr ture formed therein; a metal adjustment screw member formed with a head having at least one annular rib therein of a diameter greater than the diameter of the cylindrical aperture formed in said one end wall of the housing member, said adjustment screw member also having a threaded shank and having a conically pointed tip on the end opposite the head thereof; and a substrate member carrying an electrical resistance element, the steps including:

A. inserting the adjustment screw member, tip first, from the exterior of the housing member through the cylindrical aperture in the one end wall of the housing until a portion of the adjustment screw engages the housing and prevents further insertion;

B. placing the housing member in a housing fixture to hold the housing in a desired position;

C. inserting a second fixture into the open side of the housing member into engagement with the adjustment screw member to position the adjustment screw member in the desired position between the second fixture and one of the side walls of the housing member;

D. ultrasonically vibrating the adjustment screw member relative to the housing member while simultaneously exerting axial force between the adjustment screw and housing members to drive the head of the adjustment screw member into the cylindrical aperture in said one end wall of the housing member until the conically pointed tip of the adjustment screw member penetrates a substantial extent into the other of said spaced end walls, thereby causing the thermoplastic material of said one end wall of the housing member to soften and I flow about the rib on the head of the adjustment screw member and causing the thermoplastic material of said other end wall of the housing to simultaneously soften and flow about the conical tip of the adjustment screw member;

E. discontinuing the ultrasonic vibration and allowing the thermoplastic housing material to solidify about the region of the rib on the head and about the conical tip of the adjustment screw member whereby a front and a rear journal are simultaneously formed to rotatably support the adjustment screw member in the desired position in the hous- F. thereafter withdrawing the second fixture from the housing member;

G. assembling a resistance element wiper member with the adjustment member for movement in the housing by rotation of the adjustment member;

H. thereafter covering the open side of the housing member by assembling a substrate member, having a resistance element lying on the interior surface thereof, over the open side of the housing member with the resistance element in contact with the wiper member; and

. securing the substrate member to the housing member.

4. A method of making a variable resistor as defined in claim 3 wherein the step of securing the substrate member to the housing member includes ultrasonically deforming a portion of the housing member into engagement with the exterior surface of the substrate member lying outwardly of the interior cavity in the housing member.

5. A method of making a variable resistor as defined in claim 3 wherein the step of securing the substrate member to the housing member includes ultrasonically deforming a portion of the housing member into engagement with the exterior surface of the substrate member lying outwardly of the interior cavity in the housing member, and thereafter sealing the substrate member with the housing member by flowing and solidifying a potting agent over said exterior surface of the substrate member. 

1. In a method of makinG a variable resistor having a housing member formed with a pair of spaced end walls connected to a side wall, one of said end walls having a cylindrical aperture formed therein, and a metal adjustment member formed with a circular cross-section head, a threaded shank and a circular cross-section tip on the end opposite the head thereof, the steps including: A. inserting the adjustment member, tip first, from the exterior of the housing member through the cylindrical aperture in the one end wall of the housing; B. inserting a fixture into an open side of the housing member opposite said side wall into engagement with the adjustment member to position the adjustment member in the desired position between the fixture and the side wall of the housing member; C. ultrasonically vibrating the adjustment member relative to the housing member while simultaneously exerting axial force between the adjustment and housing members to drive the head of the adjustment member into the cylindrical aperture in said one end wall of the housing member until the circular cross-section tip of the adjustment member penetrates a substantial extent into the other of said spaced end walls of the housing member, thereby causing the thermoplastic material of said one end wall of the housing member to soften and flow about the head of the adjustment member and causing the thermoplastic material of said other end wall of the housing to simultaneously soften and flow about the tip of the adjustment member; D. discontinuing the ultrasonic vibration and allowing the thermoplastic housing material to solidify about the region of the head and about the tip of the adjustment member whereby a front and a rear journal are simultaneously formed to rotatably support the adjustment member in the desired position in the housing member; and E. thereafter withdrawing the fixture from the housing member.
 2. A method of forming a variable resistor as defined in claim 1 including the step of forming a concave groove in said side wall of the housing member for assisting in locating the adjustment member in the desired position relative thereto.
 3. In a method of making a variable resistor having a hollow housing member with an open side formed by a pair of spaced end walls connected by three side walls, one of said end walls having a cylindrical aperture formed therein; a metal adjustment screw member formed with a head having at least one annular rib therein of a diameter greater than the diameter of the cylindrical aperture formed in said one end wall of the housing member, said adjustment screw member also having a threaded shank and having a conically pointed tip on the end opposite the head thereof; and a substrate member carrying an electrical resistance element, the steps including: A. inserting the adjustment screw member, tip first, from the exterior of the housing member through the cylindrical aperture in the one end wall of the housing until a portion of the adjustment screw engages the housing and prevents further insertion; B. placing the housing member in a housing fixture to hold the housing in a desired position; C. inserting a second fixture into the open side of the housing member into engagement with the adjustment screw member to position the adjustment screw member in the desired position between the second fixture and one of the side walls of the housing member; D. ultrasonically vibrating the adjustment screw member relative to the housing member while simultaneously exerting axial force between the adjustment screw and housing members to drive the head of the adjustment screw member into the cylindrical aperture in said one end wall of the housing member until the conically pointed tip of the adjustment screw member penetrates a substantial extent into the other of said spaced end walls, thereby causing the thermoplastic material of said one end wall of the housing member to soften and flow about the rib on the head of the adjustment scrEw member and causing the thermoplastic material of said other end wall of the housing to simultaneously soften and flow about the conical tip of the adjustment screw member; E. discontinuing the ultrasonic vibration and allowing the thermoplastic housing material to solidify about the region of the rib on the head and about the conical tip of the adjustment screw member whereby a front and a rear journal are simultaneously formed to rotatably support the adjustment screw member in the desired position in the housing; F. thereafter withdrawing the second fixture from the housing member; G. assembling a resistance element wiper member with the adjustment member for movement in the housing by rotation of the adjustment member; H. thereafter covering the open side of the housing member by assembling a substrate member, having a resistance element lying on the interior surface thereof, over the open side of the housing member with the resistance element in contact with the wiper member; and I. securing the substrate member to the housing member.
 4. A method of making a variable resistor as defined in claim 3 wherein the step of securing the substrate member to the housing member includes ultrasonically deforming a portion of the housing member into engagement with the exterior surface of the substrate member lying outwardly of the interior cavity in the housing member.
 5. A method of making a variable resistor as defined in claim 3 wherein the step of securing the substrate member to the housing member includes ultrasonically deforming a portion of the housing member into engagement with the exterior surface of the substrate member lying outwardly of the interior cavity in the housing member, and thereafter sealing the substrate member with the housing member by flowing and solidifying a potting agent over said exterior surface of the substrate member. 